Recent Progress in Molecular Simulation of Flow Induced Crystallization: Biaxial Flows

Crystallization is a defining step in the fabrication of most polymeric materials. It occurs predominantly through the mechanisms of nucleation and growth from an amorphous melt or solution, which determine the ultimate morphology and properties of the resulting product. In industrial processes like fiber spinning, film blowing, extrusion, injection molding, and fused deposition modeling, crystallization takes place while the polymer is in a state of flow, which can significantly alter the crystallization kinetics, or even change the nature of nucleation and growth qualitatively. Nucleation, in particular, is very sensitive to flow-induced structures in the melt. However, nucleation occurs on time and length scales that are hard to observe experimentally. We use molecular simulations to characterize nucleation in flowing melts of linear, polyethylene-like chains, both with and without entanglements. Using nonequilibrium molecular dynamics and an analysis of mean first-passage times (MFPT), we have previously reported the dependence of enhanced nucleation rates of the structure and thermodynamics of the flowing melt in simple shear and uniaxial extensional flow. More recently, we have extended this work to include the more general cases of equibiaxial and nonequibiaxial flows. We find that a relationship describing flow enhanced nucleation as a function of orientational ordering of Kuhn segments models the kinetics most robustly for a diverse set of flow states. Furthermore, we reveal the onset of nematic ordering, previously observed for uniaxial extensional flows, in entangled polymer melts undergoing nonequibiaxial flow above a critical state of strain rate and flow asymmetry. Such nematic ordering is found to support nucleation preferentially, and is likely responsible for at least a part of the enhancement in nucleation observed in entangled polymer melts.